A majority of offshore oil and gas is produced form platforms which are erected on the seafloor and extend to above the water surface. Oil and gas is typically transported to and from these offshore platforms along pipelines laid on the seafloor. When these pipelines arrive to or depart from one of these platforms, a bend is used at the intersection of the seafloor and the platform and a vertical section of pipe or riser extends up to the platform deck. At the platform deck will be valves and piping to communicate the pipeline with equipment and/or wells on the deck of the platform.
Subsea pipelines which transport oil tend to have the waxy components of the oil come out of solution and bond to the wall of the pipeline, up to in some cases completely blocking the pipelines. In consistence, the waxy blockage appears to be exactly like black shoe polish. In most cases, a layering on the walls of the pipeline is seen and a cleaning pig passes through the pipeline to remove the waxy layering. In some cases instead of simply cleaning the layer of wax off the pipeline wall, it chips it off until it accumulates into a blockage.
When subsea pipelines which transport gas encounter the proper pressure/temperature combination and there is some water in the pipeline, a hydrate is formed. The hydrate looks a lot like crushed ice and will form quickly to create a blockage to stop the flow in the pipeline.
In either of these cases the flow of production is stopped and causes an expensive loss of production. In the case of a hydrate blockage, the operator can simply wait a few days, weeks, or months and the hydrate will melt naturally if pressure is bled off from one end or the other of the pipeline. In the case of the waxy or paraffin blockage, it is permanent until fixed.
As “fixing” these problems typically required mechanical drilling of the blockage, hot oil injection, or chemical dissolving of the blockage, communication to the blockage in the pipeline must be established. The characteristic way to establish this communication is by inserting a string of coiled tubing. A hose would work as well, but the strings of coiled tubing for up to 5 miles in length exist in the market. The hoses do not exist in the market at this length, and would be very expensive in comparison if they did.
Crawford U.S. Pat. No. 6,651,744 shows a coiled tubing string with a thruster pig (10) attached to the end for this purpose and illustrates in FIG. 12 the coiled tubing reel (150), and various other pieces of required equipment. Of note is a 5D bend section (15) which is at the intersection of the seafloor pipeline and the vertical riser. “5D” means that the bend radius of the centerline of that pipe section is 5 times the outer diameter of the pipeline. Sending equipment down the vertical pipe section around the 5D bend and then going out up to 5 miles is a very difficult goal to achieve, and is a common requirement of many offshore pipelines. If you imagine that in a 4.5 inch outer diameter pipeline this bend radius is 22.5 inches, then the steel coiled tubing must negotiate this bend.
The first problem in navigating this bend is that bending 1 to 1¾ inch outside diameter coiled tubing to a radius of 22.5 inches substantially bends and unbends the coiled tubing as it passes, both going and coming back which has a deteriorating effect on the coiled tubing. Secondly, the high loading on the inside of the pipeline bend and the outside diameter of the coiled tubing when it is being plastically bent tends to cause galling on both pipes, which destroys the integrity of whichever one(s) is galled. Thirdly, the force of pulling the coiled tubing back through the bend from its extended travel position is added to the force of pulling the coiled tubing back, pulling the pig back, and swabbing the fluid behind the coiled tubing back. At some point, you simply cannot pull the coiled tubing back and have caused a worse problem than the blockage itself.
When any or all of these factors is a concern, the only solution is to pick the pipeline up off the seafloor, saw it in half, and attach expensive connectors to the pipeline. This allows the operator to go directly into the end of the pipeline without having to pass a difficult bend. A complication to this is that before the operator can saw the pipeline in half, the pressure must be completely removed from the pipeline. Additionally, as you are sawing into a pipeline with at the least gas fumes in it, you may cause a spark.
An additional problems when going around the pipeline bends is that any equipment to pass the bend must be by definition very short or it simply will not pass and the bending of the coiled tubing tends to import moment loading on the equipment, such as the thruster pig at the end of the coiled tubing.
Offshore platforms with vertical riser pipes have been utilized in the offshore industry as long as it has existed, since the mid 1950's. Coiled tubing became available as an oilfield service tool in the mid 1960's and the marriage of these two systems happened shortly thereafter. The industry is still dealing with the problem of how to reasonably get the coiled tubing around the bends, and the most common answer today is to expensively pick the pipeline up and avoid the problem.